Along with higher integration of a semiconductor device and reduction in a chip area, miniaturization and further lamination of multiple wiring layers are being pursued. In logic circuits having multiple wiring layers, delay of signal propagating in wirings is becoming a dominant factor of restricting the operation speed of a logic circuit. Delay of signal propagating in wirings is related to a product of wiring resistance and parasitic capacitance between wirings. There is a tendency that a parasitic capacitance between wirings increases as wirings become finer. In order to suppress delay of signal propagation, it is effective to lower wiring resistance and reduce parasitic capacitance. In order to reduce parasitic capacitance, it is effective to use interlevel insulating films having a low dielectric constant. Known as insulating material having low dielectric constant are inorganic low dielectric constant insulating materials such as porous silica and SiOC, and organic low dielectric constant insulating materials such as SiLK (registered trademark) manufactured by the Dow Chemical Company. These materials contain silicon (Si) and oxygen (O), and have a lower dielectric constant than silicon oxide SiO.
In order to lower wiring resistance, techniques of using copper (Cu) having a low resistivity as wiring material in place of aluminum (Al) are practically used. It is difficult to pattern a copper layer at high precision by photolithography and etching. Damascene method is therefore adopted by which in general, a wiring trench is formed in an insulating film, the trench is filled with a copper layer, and an unnecessary copper layer on the insulating film is removed by chemical mechanical polishing (CMP).
There is a nature that copper diffuses into an insulating film. An insulating film containing diffused copper deteriorates its insulating characteristics. In order to prevent copper diffusion, a conductive barrier (barrier metal) layer of TiN, Ta or the like having a copper diffusion preventive function is formed in a wiring trench by sputtering or the like, and a copper seed layer for conducting electrolytic plating is formed on the conductive barrier layer by sputtering or the like. A copper layer as a main wiring layer is formed on the copper seed layer by electrolytic plating, and unnecessary metal layers on the insulating film are removed by CMP. An insulating copper diffusion preventive film of SiN, SiC or the like is formed covering the copper wiring.
Electro migration of an aluminum wiring is known in which aluminum atoms are migrated by current. It is said that electro migration occurs less in copper than in aluminum. However, electro migration occurs also in a copper wiring. Even if an aluminum wiring is replaced with a copper wiring, such a phenomenon occurs that a large current expected from a bulk value of copper cannot be allowed to flow. An insulating copper diffusion preventive film covering the upper surface of a copper wiring has low adhesion to the copper layer.
It is known that voids are formed at the interface between the copper wiring layer and insulating copper diffusion preventive film.
Although the barrier metal layer and copper wiring layer form a good interface, the insulating copper diffusion preventive film and copper wiring layer cannot form a good interface. It is considered that since the insulating copper diffusion preventive film and copper wiring layer provide poor adhesion, copper atoms at the interface are likely to move, and interface diffusion becomes likely to occur. It is considered that as copper atoms start diffusing at an interface with the insulating copper diffusion preventive film, copper atoms adjacent to a vacancy or vacant portion undergo volume movement, and as the number of copper atoms capable of undergoing volume movement becomes insufficient, the vacant portion cannot be filled with the moved copper atoms. Thus, voids are formed and grown.
In order to improve reliability of a copper wring, it is also known that silver, aluminum or the like may be added to copper to make this additive suppress atom flow and hence increase electro migration resistance.
As above, by adding metal atoms having a migration suppressing function to copper wiring, copper atom migration or diffusion along grain boundaries and interface between copper wiring and an adjacent insulating layer are suppressed, and electro migration resistance is improved. There is, however, a tendency that electric resistance of a wiring increases as impurity metal atoms are added to copper. For example, copper added with Al about 2 wt % has a electric resistivity increased by about 20 to 30% than that of pure copper.    [Patent Document 1]
International Publication WO2004/088745    [Patent Document 2]
JP-A-2005-38999